T1ρ mapping improvement using stretched‑type adiabatic locking pulses for assessment of human liver function at 3T
著者 奥秋 知幸
著者別表示 Okuaki Tomoyuki journal or
博士論文本文Full 学位授与番号 13301甲第4616号
Creative Commons : 表示 ‑ 非営利 ‑ 改変禁止 http://creativecommons.org/licenses/by‑nc‑nd/3.0/deed.ja
mapping improvement using stretched-type adiabatic locking pulses for assessment of human liver function at 3 T
⁎ , Yukihisa Takayamac
, Akihiro Nishied
, Tetsuo Oginoe
, Makoto Obarae
, Hiroshi Hondad
, Tosiaki Miyatib
, Marc Van Cauterena
aPhilips Healthcare, Tokyo, Japan
bDivision of Health Science, Graduate School of Medical Sciences, Kanazawa University, Kanazawa, Japan
cDepartment of Radiology Informatics and Network, Kyushu University, Graduate School of Medical Sciences, Fukuoka, Japan
dDepartment of Clinical Radiology, Kyushu University, Graduate School of Medical Sciences, Fukuoka, Japan
ePhilips Electronics Japan, Tokyo, Japan
a b s t r a c t a r t i c l e i n f o
Received 30 November 2016
Received in revised form 17 February 2017 Accepted 25 March 2017
Available online xxxx
Purpose:The purpose of this study is to investigate the performance of stretched-type adiabatic spin lock pulses for homogeneous spin locking with aﬂexible spin lock time (TSL) setting.
Methods:T1ρvalues were obtained from 61 patients andﬁve normal volunteers who were categorized using the Child–Pugh classiﬁcation and scanned using each spin lock pulse type. The pulses used were the block and two kinds of hyperbolic secant (HS); HS8_10, and HS8_5. Visual scoring was categorized using a four point scale (1:Severe, 2:Moderate, 3:Mild and 4:None) to evaluate the homogeneity of the T1ρmap and the source images obtained by each spin lock pulse. Mean T1ρvalues among the patient groups with different Child–Pugh classiﬁ- cation were compared.
Results:The visual assessment scores were 1.98 ± 1.05 for block pulse locking, 3.87 ± 0.39 for HS8_10 pulse locking, and 3.83 ± 0.45 for HS8_5 pulse locking, respectively. The scores between block pulse and HS8_10 were signiﬁcantly different (pb0.001), as were those between block pulse and HS8_5 (pb0.001).
The median T1ρvalues of normal liver function, Child–Pugh A, and Child–Pugh B or C were 37.00 ms, 40.77 ms, and 42.20 ms for block pulse, 46.75 ms, 50.78 ms, and 55.60 ms for HS8_10, and 48.80 ms, 55.42 ms, and 57.80 ms for HS8_5, respectively.
Conclusion:The spin locking sequence using stretched-type adiabatic pulses provides homogeneous liver T1ρ maps with reduced artifact and is necessary for a robust evaluation of liver function using T1ρ.
© 2017 Elsevier Inc. All rights reserved.
stretch type adiabatic pulse liver
spin lock hyperbolic secant
Assessment of liver function is essential for the management of pa- tients with liver disease and for the prevention of postoperative hepatic failure. Liver function is frequently estimated by measuring biochemical parameters in the blood, such as bilirubin, aminotransferase, alkaline phosphatase, gamma-glutamyl transferase, albumin, and prothrombin activity. The Child-Pugh classiﬁcation[2,3]is based on a combination of serum albumin, serum bilirubin, prothrombin activity, ascites, and hepatic encephalopathy to reﬂect liver functional reserve more accu- rately than by using any of these biological factors alone. Previous
reports using magnetic resonance imaging (MRI) state that T1 mapping and T2* mapping can be used to assess liver function[4,5]. Liverﬁbrosis is a common feature of most chronic liver diseases and ultimately pro- gresses to liver cirrhosis with the accumulation of proteoglycans and collagen and other macromolecules in the extracellular matrix[6–8].
Conventional MRI cannot evaluate liverﬁbrosis directly. Liver biopsy is carried out for the diagnosis and monitoring of progression as a stan- dard of reference; however, liver biopsy is an invasive procedure, is prone to error, and involves the risk of complications[9,10]. Another ap- proach, magnetic resonance elastography (MRE), is a noninvasive method for the detection ofﬁbrotic liver and staging of liverﬁbrosis [9–11]; however, additional equipment is required for MRE, especially the transducer to generate a vibrational wave.
Recently, T1ρmeasurement has been widely applied to investigate diseases of the cartilage[13–17], prostate, disc, myocardium, and liver[7,8,20–23]. With regard to the liver, Wang et al. reported that in an animal model T1ρrelaxation was used successfully to detect early liverﬁbrosis and that increased liver collagen results in an increase Abbreviations:MRE, magnetic resonance elastography; RF, radio frequency; TSL, spin
lock time; HS, hyperbolic secant; MLEV, Malcolm H. Leviti; IDL, Interactive Data Language; ROI, region of interest.
⁎ Corresponding author at: Philips Healthcare , 13-37, Kohnan 2-chome, Minato-ku, Tokyo, 108-8507 Japan.
E-mail address:firstname.lastname@example.org(T. Okuaki).
http://dx.doi.org/10.1016/j.mri.2017.03.006 0730-725X/© 2017 Elsevier Inc. All rights reserved.
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Magnetic Resonance Imaging
j o u r n a l h o m e p a g e :w w w . m r i j o u r n a l . c o m
of the T1ρvalue. Contrary to that report, Sirlin et al. suggested that T1ρrelaxation does not directly reﬂect liverﬁbrosis. Moreover, Allkemper et al. and Rauscher et al. reported that T1ρvalues were prolonged proportional to the progression of the Child–Pugh grade and liver cirrhosis[24,25].
In most of these studies, a block pulse was used as a spin lock pulse.
However, at 3 T, severe artifacts due to B0 and B1 inhomogeneity were observed, especially for a larger organ like the liver. Furthermore, these artifacts not only cause artefactual T1ρimages but also hamper accurate measurement of the T1ρ. Such artifacts are caused by inhomogeneous B0 and B1 and therefore imperfect excitation pulses in the spin lock pulse train. Witschey et al.reported an improved spin locking pulse, namely the echo locking method, to remediate these artifacts in a phan- tom and the human brain but not in the liver.
Adiabatic pulses are characterized by the simultaneous modulation of radio frequency (RF) wave amplitude and frequency. Well-chosen modulation wave forms result in insensitivity to a broad range of B0 and B1 inhomogeneity[27–31].
A few papers reported application of adiabatic pulses for spin locking. Taheri S et al. and Michaeli S reported a simulation study[32, 33], Mangia S et al. studied the brain at 4 T, Casula V et al. studied knee cartilageand Yang Q et al. applied them to liver.
The purpose of this study was to investigate the performance of the stretched-type adiabatic spin lock pulses for homogeneous spin locking with aﬂexible spin lock time (TSL) setting. Moreover, we aimed to in- vestigate the clinical usefulness of the T1ρvalue acquired using the im- proved spin locking pulses for the assessment of liver function.
2. Materials and methods 2.1. Subjects
This study was approved by our Institutional Review Board; it com- plied with the standards of the Ethics Committee.
Between June 2013 and July 2014, 69 patients underwent MRI of the liver, including T1ρmaps, because a liver tumor was suspected due to chronic liver disease or malignant disease in other organs. We excluded eight patients with difﬁculty to determine Child–Pugh classiﬁcation. Of the remaining 61 patients, 51 patients were categorized as Child–Pugh A (age range, 40–83 years; mean age, 63.4 years) and 10 as Child– Pugh B or C (age range, 43–74 years; mean age, 61.3 years). As the con- trol, ﬁve normal volunteers (age range, 50–55 years; mean age, 52.6 years) were scanned.
2.2. Imaging protocols
MRI data acquisition was performed on a 3 T clinical scanner (Achieva TX; Philips Healthcare, Best, Netherlands). A 32-channel phased-array receiver coil was used.
Three types of rotary echo spin lock pulses were generated to achieve T1ρ-weighted images (Table 1). The non-selective block pulse was used as a conventional spin lock pulse (Fig. 1). The spin lock pulse amplitude was set at 500 Hz, following the literature[20,36]; the TSLs were 1, 20, and 40 ms. Two different types of spin lock pulses type
were used, namely the block pulse and the stretched-type adiabatic preparation pulses.
The hyperbolic secant (HS) pulse, known to be insensitive to varia- tions of B1 intensity approaching several orders of magnitude, was used as a base. The time variation of the wave form modulations was modiﬁed to further optimize the performance of the pulse.
An effective magneticﬁeld (ωeff=γBeff) is induced by the pulse, whereBeffis the effective magneticﬁeld vector andγis the gyromagnet- ic ratio. The time-dependent magnitude of the effectiveﬁeld is:
ωeffð Þ ¼t ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ω21ð Þ þt ðω0−ωRFð Þt Þ2 q
ω1(t) =ω1maxsech(βtn), (ω0−ωRF(t))= A∫sech2(βτn)dτ, whereω1max
is the maximum amplitude. A determines the frequency sweep amplitude and is equal toμβ.μis a dimensionless parameter andβis a modulation angular frequency.ω0is the on-resonance fre- quency equal toγB0andωRFis the frequency modulation. For the orig- inal HS pulse, n equal to 1, whereas the stretched adiabatic pulse is generated bynN1 (Fig. 2). We applied a high n factor (n= 8) to gener- ate two types of adiabatic spin locking pulse from the original pulse, which was characterized byμ= 5 andβ= 4 rad/s. The parameters of theﬁrst pulse type were pulse duration 10 ms, frequency sweep 636.6 Hz, and maximum amplitude 6.37μT (HS8_10). The second pulse type had a pulse duration 5 ms, frequency sweep 1273.2 Hz, and maximum amplitude 13.48μT (HS8_5). The Beffchange during the two types of adiabatic pulse is indicated inFig. 3; the average Beffﬁeld of HS8_10 and HS8_5 was 500 Hz and 1000 Hz, respectively. For these adiabatic pulses, the rotating frame relaxation time was measured using a pulse module consisting of consecutive identical HS pulses.
The number of pulses was increased to obtain the T1ρimages with dif- ferent TSLs forﬁtting from two to four for HS8_10 and from four to eight for HS8_5, with MLEV phase cycling(Fig. 4). The TSL was set to 0, 20, and 40 ms by increasing the number of pulses. TSL = 0 rep- resents the absence of a spin lock pulse.
Design parameters for the spin lock pulses.
Spin lock pulse Pulse type
Pulse duration per pulse
(ms) Frequency sweep (Hz)
Spin lock frequency (Hz)
Block Block – – 500 11.7 1,20,40
HS8_10 Stretched-type adiabatic (n= 8) 10 636.6 500.0* 6.73 0,20,40
HS8_5 Stretched-type adiabatic (n= 8) 5 1273.2 1000.0* 13.48 0,20,40
*average effective SL frequency.
Fig. 1.Scheme of the T1ρspin lock RF cluster using block pulse. The RF pulse train is 90(+x)–SL(+y)–SL(−y)–90 (−x). Magnetization is nutated into thexaxis by the initial 90° pulse. The magnetization vector is then spin locked by two block pulses, lockingﬁrst along the +yand then the–yaxis. Finally, the magnetization is nutated back into thezaxis by the 90° pulse along the–xaxis.
We simulated the efﬁciency of these three types of spin lock pulse in function of B0 and B1 using Bloch equations at TSL 20 ms (Fig. 5). The simulation was based on the Bloch-equations, solved using the ordinary differential equation function of Matlab (Mathworks, Natick, MA, USA).
We did not take relaxation effects into account. Theﬁgures show the ﬁnal magnetization, in function of B0 and B1, after presenting each type of spin lock pulse combination to the equilibrium magnetization.
The spin locking block pulse combination nutates the equilibrium magnetization into the x axis by the initial 90° pulse and is then spin locked by two block pulses that have opposite phases. After the spin locking the magnetization is nutated back into the z axis by a−90°
For the two types of stretched-type adiabatic pulse, the magnetiza- tion is spin locked by an even number of adiabatic pulses (Fig, 5 b and c).
In case of perfect locking, and ignoring relaxation, theﬁnal magneti- zation is equal to the equilibrium magnetization.
In theﬁgures we show theﬁnal magnetization as a fraction of the equilibrium magnetization, in function of both B0 and B1.
For imaging, a three-dimensional turboﬁeld echo sequence using the parallel imaging technique was employed with breath holding.
Volume shimming was used to minimize B0 inhomogeneity. Other im- aging parameters were as follows: repetition time 2.1 ms, echo time 0.98 ms,ﬁeld of view 360 × 306 mm2, matrix 256 × 205, slice thickness 10 mm, slice gap 0 mm, number of slices 3, number of acquisitions 1, shot interval 5000 ms, and scan time of T1ρ-prepared image with each TSL 11.7 s. Scanned slices were set at the level of the hepatic hilum.
2.3. T1ρimage analysis
T1ρmaps were generated on a pixel-by-pixel basis using in-house developed software program written in Interactive Data Language (IDL 6.3; ITT, Boulder, CO) using a mono-exponential decay model:
M(TSL) = M0 × exp.(−TSL/ T1ρ), where M0 and M(TSL) denote the equilibrium magnetization and T1ρspin lock prepared magnetization for each TSL, respectively. T1ρmaps were generated for each spin lock type with a Levenberg–Marquardtﬁtting algorithm. For quantiﬁca- tion of liver T1ρvalue, three regions of interest (ROIs) were manually placed on the liver parenchyma; the sizes of the ROIs were as large as possible, approximately 50–150 mm2, avoiding blood vessels, tumors, and artifacts.
2.4. Visual assessment
Evaluation of the homogeneity of the T1ρmaps and the T1ρsource images was scored through visual evaluation by two MR clinical scien- tists (T.O. and M.O.) with 10–16 years of experience. Visual scoring was categorized using a 4-point scale as 1 severe, 2 moderate, 3 mild, and 4 none. The deﬁnition of visual scoring was as follows:
1: Severe: Severe artifacts, difﬁcult to evaluate T1ρvalues;
2: Moderate: Mildly severe artifacts. Artifacts should be avoided to measure the T1ρvalue, however, theﬁtting to calculate T1ρvalues is acceptable using a limited area;
3: Mild: Some artifacts on the image;
4: None: No artifacts on the image.
2.5. Statistical analysis
We employed the Tukey–Kramer method to compare visual scoring between the image qualities and the Friedman test to compare the mean T1ρvalues of each spin locking pulse (the block pulse, HS8_10, and HS8_5).
Mean T1ρvalues among the patient groups with different Child– Pugh classiﬁcations were compared using the Kruskal–Wallis test followed by Tukey's post hoc test. Statistical analysis was performed Fig. 2.Scheme of the amplitude and frequency modulation function for HS1 and HS8. HS8
is the stretched-type version of HS1. Time is shown on the horizontal axis.
Fig. 3.The amplitude, frequency and effective frequency change during two types of adiabatic pulses: HS8_10 a) and HS8_5 b).
using EZR free software (Saitama Medical Center, Jichi Medical Univer- sity, Saitama, Japan). APvalueb0.01 was considered signiﬁcant.
The simulation suggested that the HS8_5 pulses would perform bet- ter as a locking pulse, given that for a broad range of B0 and B1 values the equilibrium magnetization is restored after the locking.
Two cases of typical source images and T1ρmaps are shown inFig. 6a and b. There were artifacts on most of the spin locking images using the block pulse; however, the artifact level on the source images and T1ρ mapping obtained with the adiabatic spin locking pulses was signiﬁ- cantly reduced.
3.1. Visual assessment
The intraclass correlation coefﬁcient of visual scoring between the two readers was 0.90, indicating good concordance in the readers' visual scoring.
The visual assessment score (mean ± standard deviation) of the ho- mogeneity of the T1ρmaps resulted in 1.98 ± 1.05 for block pulse locking, 3.87 ± 0.39 for HS8_10 pulse locking, and 3.83 ± 0.45 for HS8_5 pulse locking, respectively. Both types of adiabatic spin locking- derived maps scored signiﬁcantly better than the block pulse locking- derived maps (Fig. 7). The visual assessment score showed signiﬁcant differences between the block pulse and HS8_10 (Pb 0.001) and
between block pulse locking and HS8_5 (Pb0.001). There was no signif- icant difference between HS8_10 and HS8_5 (P= 0.792).
It was not possible to draw an ROI avoiding the artifacts in all source images for some T1ρmaps using block pulse because artifacts appeared in different locations in each source image. This resulted in a limited area where the T1ρvalue could be calculated to generate a reliable T1ρ map. In contrast, there were no severe artifacts in most of the source im- ages using adiabatic pulses.
The T1ρvalues calculated using each spin lock type are shown in Table 2. The mean T1ρvalues for normal tissue of block, HS_10, and HS_5 were 37.07 ms (range: 32.47–42.41 ms), 46.12 ms (range:
44.03–48.77 ms), and 49.99 ms (range: 47.97–51.43 ms), respectively.
T1ρvalues were signiﬁcantly different between each spin lock type (Friedman test,Pb0.0001).
The T1ρvalues for each spin lock type are shown inFig. 8. The median T1ρvalues of normal liver function, Child–Pugh A, and Child–Pugh B or C were 37.00 ms, 40.77 ms, and 42.20 ms for block pulse; 46.75 ms, 50.78 ms, and 55.60 ms for HS8_10; and 48.80 ms, 55.42 ms, and 57.80 ms for HS8_5, respectively.
T1ρ values were not signiﬁcantly different between normal liver function and Child–Pugh B or C using block pulse (Kruskal–Wallis, P= 0.038). We found a signiﬁcant difference between normal liver function and Child–Pugh B or C using the HS8_10 pulse as well as the HS8_5 pulse, with a high signiﬁcance level (Kruskal–Wallis,Pb0.01).
Fig. 4.Scheme of the amplitude and frequency modulation function for HS8_10 and HS8_5 locking of 20 ms duration. HS8_10 locking consists of two segments a) and HS8–5 consists of four segments b), with an MLEV-type phase cycle.
Fig. 5.Simulated longitudinal magnetization after each spin lock pulse at TSL 20 ms: a) block pulse, b) HS8_10, and c) HS8_5. Relative B1 amplitude is on the vertical axis, andΔB0 is on the horizontal axis, in kHz units. Theﬁnal longitudinal magnetization after locking with HS8_10 and HS8_5 is shown to be more homogeneous for a wide range of relative B1 amplitude and ΔB0 compared to using block pulses. The contour lines in the plot are per 0.2 Meq interval, with dark blue equal to–Meq and yellow equal to +Meq. (For interpretation of the references to colour in thisﬁgure legend, the reader is referred to the web version of this article.)
We investigated the effectiveness of applying a module consisting of stretched adiabatic pulses as a locking pulse to acquire robust T1ρmaps at 3 T. Using the block pulse, which is standard in most spin locking
sequences for whole body systems, we found severe artifacts on the source images. The use of adiabatic pulses reduces B1 inhomogeneity- related artifacts, which cause fatal errors in calculating T1ρvalues.
Witschey et al. reported a spin locking pulse that mitigates B0- and B1-type artifacts at allﬁeld strengths. This type of B0 and B1 Fig. 6.Source images for each TSL and the calculated T1ρmaps. The typical source images for each spin lock type are shown: block pulse (top), HS8_10 (middle), and HS8_5 (bottom) with the resulting T1ρmap (right). Using the block pulse, there are severe artifacts in segment 3 and segment 4 on each TSL image (white arrow) a), and severe artifacts in segment 7 and segment 8 on each TSL image (white arrow) b).
insensitive spin lock pulse (90(x)–τ/2(y)–180(y)–τ/2(−y)–90(x)) was used for a neurological study. However, in our experience, it is not well suited for the liver due to the size of theﬁeld of view of the liver compared to the brain and the heart.
The diagram of theﬁnal longitudinal magnetization simulated using the Bloch equations after presenting each type of spin lock pulse to the equilibrium magnetization clearly shows that the block pulse is very sensitive to B0 and B1 inhomogeneity (Fig.5). This will cause severe ar- tifacts on the image. In contrast, adiabatic spin locking results in a ho- mogeneousﬁnal magnetization for a wide range of B0 and B1ﬁelds. In particular, HS8_5 maintains homogeneity; therefore, it is useful for big organs, like the liver, where B0 and B1 are inhomogeneous, especially
at 3 T and higherﬁeld strengths. However, there is no signiﬁcant differ- ence in visual scoring between HS8_10 and HS8_5. Thus, we can con- clude that the HS8_10 is working as well as the HS8_5 within the range of B0 and B1 variations expected for a liver examination on a clin- ical 3 T system like the one we used. The difference in visual scoring be- tween the block and the two types of adiabatic spin locking pulse corroborates theﬁndings from the simulation. The stretched-type adia- batic spin locking pulse was able to provide homogeneous T1ρimages.
Spin locking using adiabatic pulses represents a promising improve- ment to overcome B1 and B0 inhomogeneity, especially for body T1ρ imaging.
Both mean T1ρvalues obtained from the block and the two types of adiabatic spin locking pulse increased as liver function worsened. This result was similar to those previously reported[20,21,24]. However, the fundamental mechanisms leading to T1ρprolongation in liverﬁbro- sis and cirrhosis have not been fully investigated. Wang et al.
described the usefulness of T1ρrelaxation for the evaluation of liverﬁ- brosis in a rat biliary duct ligation model. Takayama et al.claimed that T1ρrelaxation of the liver was not signiﬁcantly correlated with liverﬁbrosis or with necro-inﬂammation. Thus, they concluded that T1ρhas potential as a biomarker for the assessment of liver function, al- though it may not be suitable to estimate liverﬁbrosis or necro-inﬂam- mation. The present study did not investigate the mechanisms of T1ρ prolongation with liver dysfunction. Liver dysfunction may contribute to T1ρprolongation through combined mechanisms, such as biological, chemical, and physical factors, as Takayama et al. suggested.
The obtained T1ρvalues using the adiabatic pulses were longer than those using the block pulse, similar to the results obtained by Jokivarsi et al.in normal rat brain parenchyma, even if the average Beffis equal, as for the HS8_10 pulse. We conjecture that this difference in T1ρis because the Beffof the adiabatic pulse is continuously changing during the pulse (Fig. 3), in contrast to the block pulse. The HS8_5 has a higher average Beff, therefore a different T1ρcan be expected[32,42].
Furthermore, temporal and spatial variation of Beffwill result in a vary- ing T1ρrelaxation rate, which is difﬁcult to control or predict. One must be aware of this when interpreting the T1ρvalues, and this should thus be done with extreme caution. Moreover, liver tissue contains a variety of macromolecules causing a mix of T1ρvalues within the voxels. Fur- ther, it is known that exchange and diffusion effects occur during Fig. 7.Results of visual scoring. The asterisk (*) represents a value ofPb0.001 between
block and HS8_5 and between block and HS8_10. There is no signiﬁcant difference between HS8_10 and HS8_5 (P= 0.792).
Difference in mean T1ρvalues and range for normal tissue between each spin lock type.
Spin lock pulse Block HS8_10 HS8_5
Mean T1ρ(ms) Range of T1ρ(ms) Standard deviation (ms)
Fig. 8.Comparison of T1ρvalues between block, HS8_10, and HS8_5. The graphs indicate mean liver T1ρvalues and standard deviation of Child–Pugh A, Child–Pugh B or C, and normal using each spin lock pulse. Each spin lock type is shown on the horizontal axis; T1ρvalues are on the vertical axis.
locking and can affect theﬁnal signal. We did not study this aspect with- in the scope of this project.
It is thus difﬁcult to achieve a meaningful absolute quantitative T1ρin the clinical setting because the Beffvaries temporally and spatially. How- ever, using adiabatic locking pulses provides a locking that is effective over a largeﬁeld of view and thus enables the calculation of homoge- neous T1ρmaps. The visual scores of the T1ρmaps acquired with adia- batic pulse locking are indeed higher compared to traditional block pulse locking. The visual scores for the respective Child–Pugh grades were not signiﬁcantly different between HS8_10 and HS8_5. Further- more, the use of HS8_10 and HS8_5 pulses allows to differentiate be- tween normal and Child–Pugh B or C with high statistical signiﬁcance.
There is a limitation in the present study. The pulse duration of the adiabatic pulse was determined by the modulation function and opti- mal adiabatic condition. The total locking pulse is limited to an even number of adiabatic pulse segments (Fig. 4), and thus the TSL duration was restricted to an even multiple of that segment duration. In this study, HS8_10 uses 10 ms per adiabatic pulse segment, and the TSLs were set to 20 ms and 40 ms. However, we believe that this restriction to the TSLs was not disadvantageous for the calculation of the T1ρ value of the liver, as mentioned in previous research[20,21,24].
In conclusion, we demonstrated that the stretched-type adiabatic spin locking method provides homogeneous and reduced artifact T1ρ liver images and is useful for a robust evaluation of liver function using T1ρ.
Disclosure of conﬂicts of interest
The authors have a conﬂict of interest to declare. Tomoyuki Okuaki and Marc Van Cauteren are Philips Healthcare employee. Tetsuo Ogino and Makoto Obara are Philips Electronics Japan employee. Yukihisa Takayama, Akihiro Nishie, Hiroshi Honda and Toshiaki Miyati have no relevant conﬂicts of interest to disclose.
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